Devices are commonly coated with thin films and other coatings in order to enhance their performance and functionality. Such coatings can be broadly characterized as being either hard coatings or soft coatings. Hard coatings, such as ceramic and diamond-like carbon, for example, are often applied to cutting tools to enhance their cutting ability and durability. Soft coatings, such as polymer-based materials, for example, are often applied to medical devices to improve their bio-compatibility.
Scratch tests are often performed on such coatings to study the mechanical behavior of the surface in terms of wear resistance or resistance against scratching. When performing such scratch tests, a scratch or indenter probe is pressed against the surface of the device with a normal force (i.e. perpendicular to the surface) and moved across the surface of the device, thereby creating a so-called lateral or coaxial force (i.e. parallel to the surface in the direction of probe movement) to scratch the coating. In some instances, the normal force is a constant force, while in other instances, the normal force is “ramped up” (e.g. in a linear fashion) as the indenter probe is moved across the surface.
When ramping the normal force while scratching the surface, the coating along the scratch track typically displays a fully elastic deformation regime, followed by a plastic deformation regime, and finally fracture. The transition from plastic deformation to fracture (the point of fracture) indicates a critical normal force which is used to rate the performance of the surface. In order to identify the point of fracture, conventional techniques often study the surface friction (i.e. ratio of coaxial force to normal force), or study the coaxial force in combination with visual observations of the scratch track.
However, due to a changing coefficient of friction, it is often difficult to determine the fracture point using friction analysis. Also, fractures in the coating are often too small to accurately identify, even with a microscope, while “scattering” or changes in the coaxial force are often inconsistent. Other techniques sometimes used to identify the fracture probe include study of normal displacement of the indenter probe (i.e. vertically relative to the surface) and acoustical transmissions. In any case, however, each of these approaches often provides inconsistent results and, as such, do not always successfully provide a reproducible pattern of fracture starts between multiple samples of a same device.